The present invention relates to a degassing apparatus for a metal mold which degasses a cavity of the metal mold during injection molding by an injection molding apparatus, a die-cast machine or the like.
In a conventional injection molding apparatus, for example, a die-cast machine, when a molten metal is charged in the mold cavity at high speed and high pressure, gas in the mold cavity often cannot be sufficiently removed and may become mixed with the molten metal used to form a product, thus forming a void in the molded product.
U.S. Pat. No. 4,431,047 ('047 patent) describes a degassing apparatus for a metal mold which can remove a large volume of gas within a short period of time. The degassing apparatus comprises a vent groove formed on the dividing or mating surfaces of the metal mold. The vent groove communicates with the mold cavity.
The apparatus described in the '047 patent also includes a valve having a reciprocatively movable valve body, and a bypass conduit which provides an exhaust path from the mold cavity. The bypass conduit joins the vent groove midway along its length and connects with the valve. The valve body may be moved between an open and closed position. In the open position, the valve body allows gas from the bypass conduit to pass freely through the valve. In the closed position, the valve body blocks the bypass conduit and the vent groove so that no molten metal may pass into the valve. The valve body is positioned in line with the vent groove at an end of the groove that is distal from the mold cavity.
The degassing apparatus of the '047 patent allows gas in the mold cavity to escape during injection molding through the bypass conduit and the valve. When molten metal is charged in the cavity and reaches the end of the vent groove, the molten metal has sufficient mass to push the valve body from the open position to the closed position, whereupon the bypass conduit and vent groove are closed so that no molten metal can escape.
Although the degassing apparatus described in the '047 patent works satisfactorily in many applications, it has a number of inherent drawbacks.
More specifically, in the conventional degassing apparatus for a metal mold described above, when the flow speed of the molten metal into the vent groove is low or the flow amount is small, thus providing a small inertia force, the valve is not often completely closed. In addition, since a vacuum suction device is used, part of the molten metal undesirably often flows to the vent path before the molten metal sufficiently acts on the valve body. In this case, the molten metal forms splashes or droplets and attaches to and is solidified on the tapered portion on the outer surfaces of the upper portion of the valve head or the valve seat. Then, the valve cannot be fully opened, the molten metal flows into the valve chamber and damages it, or the molten metal flows out from the valve chamber.